<!DOCTYPE html>
<html>
<style>

body {
  height: 10000px;
}

.outer-opacity {
  opacity: 0.8;
}

.scrollbox {
  border: 1px solid black;
  width: 200px;
  height: 400px;
  overflow: hidden;
}

.inner-opacity {
  height: 1000px;
  opacity: 0.8;
}

.fixed {
  background: blue;
  width: 100px;
  height: 100px;
}

</style>

<div class="outer-opacity">
  <div class="scrollbox">
    <div class="inner-opacity">
      <div class="fixed"></div>
    </div>
  </div>
</div>
</html>
